This invention relates generally to bonding wires, more particularly, the welding of very fine wires to metal surfaces. More specifically, the invention is directed to an improved microwelding apparatus and process for joining very fine wires with diameters less than 3 mils to terminals of electronic elements.
Electronic packaging of integrated circuit semiconductor devices has become increasingly more microminiaturized and has placed greater demands on the circuit joining technology involved. Terminals on integrated circuit devices that may number in the hundreds must be joined to metallurgy pads on supporting substrates. Also, the substrates of the packages have become increasingly more complex, often supporting as many as 130 integrated semiconductor devices and making the necessary circuit interconnection wiring. In view of the complexity and high cost of the substrate, it is frequently necessary to make engineering changes in the circuitry and also correct for defects in the interconnection metallurgy. The connections necessary to implement these objectives can be conveniently made using very fine wires bonded to selected pads on the device and/or substrate.
The most common fine wire bonding technique is called ultrasonic bonding. In this process the end of the wire to be bonded is positioned under a bonding tip by means of a wire guide hole in the tip. A load is exerted on the tip which causes the wire to be forced against the substrate land or against the chip pad. A horizontal cyclic displacement of the tip relative to the substrate is produced at an ultrasonic frequency for a predetermined length of time. This ultrasonic motion scrubs the wire-land interface surfaces, fragmenting oxide films and any other films to produce a clean-metal-to-clean-metal contact at the interface surfaces and produces a shallow room temperature diffusion and alloying of the surface metals. This results in a metallic bond between the wire and the land or pad. It is believed that the frictional heat developed by the ultrasonic motion at the interface surfaces probably results in diffusion occurring at somewhat higher than room temperature.
The ultrasonic bonding technique has a major disadvantage which is very significant, particularly when used to make changes or correct for defects in the metallurgy system of a multilayer ceramic (MLC) substrate, of the type described and claimed in U.S. Pat. No. 4,245,273. The ultrasonic bonding technique requires that the bonding pad or surface be formed of a heavy gold layer with a thickness of at least 10 micron. In the fabrication of such MLC substrates, this heavy gold layer can only be deposited by electroplating. This requires a series of additional steps which increase the cost, as well as subjects the substrate to additional environments which reduce the yield.
It is known that fine wires can be bonded by microwelding techniques as disclosed in U.S. Pat. No. 4,171,477. In this technique a pair of electrodes which are electrically isolated from each other are positioned over a wire in contact with a metal surface. A voltage source applies a voltage between the electrodes which causes a current to flow through the tips and through a very short segment of the wire in contact with the electrodes. This produces a momentary heating of the wire so that it melts and fuses a wire segment to the metal surface. This type of bond does not require the heavy gold layer that is necessary in ultrasonic bonding. However, the resultant welds vary in quality and strength, and the welding tips have a relatively short life.